Cosmo's Treatise on Detonation

Intro

A lot of engine modifications for more torque and horsepower have to do with packing more air and fuel into the cylinders: intake, exhaust, porting, turbocharging, intercooling, injectors, fuel pump, etc. This is largely a problem of fluid dynamics, the various solutions to which involve mostly mechanics and electronics, limited only by the modifier's budget and ingenuity; but it is important to understand that there is at least one hard limit imposed by chemistry. That limit is Detonation.

This treatise is a summary of my understanding, as an amateur, of detonation. Sources of learning on this subject have included various magazine articles, discussions with other performance enthusiasts, other articles on the web, and the Toyota 1993 MR2 Repair Manual (BGB).

Disclaimer

What follows is a summary of the knowledge that I have accumulated while researching the Toyota MR2 Turbo, and automotive performance in general. Some of my sources are more authoritative than others. I am sufficiently satisfied as to the completeness and veracity of my understanding of the subject that I will modify my own MR2 Turbo based on what I know, with reasonable confidence that I can avoid unexpected engine damage or safety issues. I present the following only in case it might guide the researches of other enthusiasts. I make no claim that any of the following is true or correct; I am not suggesting that anyone make the changes here discussed to an automobile or similar system; I take no responsibility for the consequences of any actions taken by anyone based on my statements. I do not condone nor suggest illegal activity with respect to The Highway Traffic Act or any other laws.

I am not affiliated with any automobile manufacturer, seller or repair shop, nor any parts manufacturer, supplier, seller or installer, nor any automotive publication. I am in no way associated with the automotive industry except in that I own an automobile, and I declare that I am not an expert in any way. The opinions that I express in relation to automobiles and their performance do not necessarily reflect those of my employer, which is similarly not affiliated with the automotive industry.

Good and Bad Combustion

Normal combustion in an engine is slow. The air/fuel mixture is compressed into the combustion chamber, the spark plug ignites it, a flame front travels through the mixture from the spark plug outwards, like an expanding balloon, to the edges of the cylinder. After combustion, the gases are at a much higher pressure than before. This higher pressure acts on the piston, pushing it down and supplying power. This is slow, controlled combustion. It may seem strange to think of normal combustion, which happens thousands of times per second in each cylinder, as slow, but in comparison to a detonation event, it is.

Detonation is a much more explosive form of combustion. It is caused by the temperature and pressure of the unburnt air/fuel mixture getting too high. It usually happens after the spark plug has fired. As the flame front expands outwards from the plug, it increases the pressure in the unburnt gases ahead of it, around the outer edge of the chamber. The temperature and pressure of the remaining unburnt mixture goes beyond the critical point, and it explodes all at once. This detonation causes a violent spike in cylinder pressures, much higher than is experienced with normal combustion. The pressure spike is what causes engine damage, and it can be heard from outside the engine as a "ping" or "knock" sound.

Unfortunately, detonation can also be caused by "hot spots" in the combustion chamber. These can be bits of carbon build-up, or spark plugs of improper heat range, or even scratches on the piston face.

The Chemistry Of Detonation

Detonation is caused by a combination of too-high temperature and pressure. It is possible to avoid detonation by decreasing one as you increase the other. As most engine mods result in more air/fuel being pushed into the cylinders, therefore creating higher cylinder pressures, it is necessary to control or decrease the temperature of both the incoming mixture and of the engine itself.

The critical point of detonation will be different for every type of gasoline. The octane rating is a measure of the gasoline's resistance to detonation: the higher the better. Higher-octane gasoline is not "more powerful" gasoline, as many people think; but, because it is more resistant to detonation, you can extract more power from your engine with higher-octane gasoline. Detonation control is the reason that manufacturers specify only high-octane gasoline for their high-performance models.

The air/fuel ratio of the mixture is very relevant. A perfect mixture (stoichiometric) makes the most efficient use of engine displacement; it also burns the hottest. Unmodified engines run a little rich (more fuel than air) for safety. The excess gasoline cools the air/fuel mixture before combustion, and releases less heat during combustion, leaving the cylinder, piston and head cooler after combustion. A lean mixture (more air than fuel) burns fast and hot, and is more prone to detonation.

Factory Detonation Avoidance

Auto manufacturers avoid detonation in the following ways.

As already noted, stock engines run with a slightly rich air/fuel mixture.

The temperature of the engine is maintained at a safe level by the cooling system (coolant, radiator, fan, water pump, thermostat).

The spark timing is set such that normal combustion will complete during the downstroke of the piston (the "power stroke"), which relieves the pressure somewhat.

Some engines have a knock sensor, which is like a microphone bolted to the outside of the engine. If the knock sensor detects detonation, the ECU can retard spark timing to reduce peak cylinder pressures.

In the case of some factory turbocharged cars, including the MR2 Turbo, the ECU has the ability to limit boost under high-risk conditions. High boost makes high cylinder pressures and high demands on the fuel system.

The EGR (Exhaust Gas Recirculator) system is a pollution-control system. It recirculates exhaust gas back into the intake stream, for the purpose of cooling combustion, which reduces NOx emissions. The EGR system does not operate at WOT, so peak engine output is not affected.

When you increase performance by packing more air/fuel into the cylinders, you increase the pressure of gases in the cylinder before the spark plug fires; you might also increase the temperature of those incoming gases (especially with supercharging), as well as the temperature of the engine itself. The result of which is that you increase the chances of detonation. Unless you can afford to rebuild or replace your engine on a regular basis, it is important that all performance upgrades are made in concert with detonation avoidance measures.

Mild Detonation Avoidance Measures

Before modifying an engine, the enthusiast should ensure that all systems that affect detonation are in top working condition. The cooling system should be inspected, flushed and re-filled with premium coolant. Ensure that nothing impedes the flow of cooling air through the radiator and engine compartment. Flow can be encouraged by installing electric fans against the bottom of the vents in the engine lid on the MR2 (blowing outwards).

Only premium, high-octane gasoline should be used. Premium gasolines from the major oil companies contain additives that will keep injectors clean of deposits (which can affect fuel delivery), and the combustion chamber clean of carbon build-up (which creates hot spots). Injector cleaner should be used regularly. The fuel filter should be changed, and changed often.

If the engine is equiped with a knock sensor, then the ECU will be able to deal with mild detonation by retarding spark timing. The MR2 Turbo has such an arrangement. The base timing is set mechanically with distributor adjustment, and is advanced or retarded electronically by the ECU. If this mechanism is relied upon too heavily, then the chronically delayed spark can burn distributor contacts, and cause ignition problems. This can be avoided by retarding the base timing, by turning back the distributor a few degrees.

Colder heat range spark plugs should be used, and the plugs should be inspected regularly for signs of burning, fouling or detonation, to ensure that the correct heat range is being used. For a photographic guide to reading sparkplugs, refer to the FAQ on the NGK web page. Or, refer to a good book on automechanics.

Maximizing the efficiency of the intercooler, if the engine is so equiped, helps to reduce the temperature of the intake air. The intercooler and intake plumbing should be removed and cleaned of oil and grease; soapy hot water does the job. Steps should be taken to prevent the entry of oil into the intake from the PCV circuit; this can include a PCV filter or a catch-can mod. The flow of cooling air through the intercooler should be increased, perhaps by attaching an electric fan to it. Note: the PCV circuit is an emissions control system. It is illegal in some states and countries to modify this system for use on public roads.

The most important measures are to protect the operation of the fuel system. An undetected lean burn condition can lead to overheating and severe detonation in a matter of seconds. Ensure that performance mods that increase maximum airflow do not exceed the capabilities of the fuel system. As already noted, the fuel filter should be changed, and changed often. On a higher-mileage car, the fuel injectors should be cleaned and balanced.

Monitoring for Detonation

Detonation can sometimes be heard as a "pinging" or "knocking" sound. It has been described as the sound of shaking a bean in a coffee can. One should listen for this, especially during high-boost, WOT operation.

The spark plugs should be inspected regularly for signs of poor gasoline, incorrect fuel mixture, inappropriate heat range, or detonation.

There are some gauges which can give a more immediate view of the conditions that can cause detonation:

Fuel pressure gauge

If the demand for fuel exceeds the capacity of the pump, then the fuel pressure will drop, resulting in reduced fuel delivery and a lean burn condition.

Exhaust Gas Temperature (EGT) gauge

The temperature of the exhaust gases in the exhaust manifold is directly related to the temperature of the pistons and combustion chamber. Higher temperatures increase the probability of detonation. Based on the engine type, air/fuel ratio and other factors, there is a temperature that should not be exceeded. This temperature will be in the neighbourhood of 900 degrees Celsius (1650 degrees Fahrenheit), but the enthusiast should consult an expert performance tuner for an authoritative number. One limitation of the EGT gauge is that critically high temperature in only one cylinder (due to unequal air or fuel delivery) may not be detected, depending on the number and position of temperature probe(s).

Air/Fuel Ratio (A/F) gauge

This gauge displays the voltage on the oxygen sensor (one of the factory emissions control devices). Because of the very non-linear nature of factory oxygen sensors, an A/F gauge can not read the absolute ratio, but can only indicate increases or decreases in the ratio. Performance tuners will sometimes use a "wide-band oxygen sensor" while on a dyno to fine-tune the fuel delivery mapping of an engine. These devices are very costly and are not generally permanently installed on a vehicle.

Stronger Detonation Avoidance Measures

As performance continues to increase, more steps must be taken to control detonation. The goals of these stronger measures remain the same: control temperature of engine and of intake air, provide sufficient fuel, set appropriate spark timing.

bigger intercooler or air/water intercooler

spray of water or compressed gas on outside of air/air intercooler

bigger turbo (adds less heat to intake air for a given boost pressure)

water mist system (sprays water mist into intake; high "heat of evaporation" of water controls temperature of air/fuel mixture during compression)

*Some of these modifications may necessitate changes to the engine control system, either by ECU reprogramming or by piggyback computers.

A Word About Nitrous Oxide

Nitrous oxide is an oxidant which is more oxygen-rich than air; therefore allowing more fuel to be delivered and burned, therefore producing more power. Some nitrous oxide systems ("dry" systems) rely on the stock fuel system to deliver enough fuel for the air and NOx being consumed by the engine, while other ("wet") systems inject NOx and additional fuel together in the correct proportion. In either case, it is vitally important that sufficient fuel be delivered when the NOx system is operational. Detonation can be sudden and severe when NOx is concerned. If a spray of NOx onto the outside of the intercooler is used to cool it, then the intake must be protected from this unmetered source of oxidant. Engine temperatures (coolant, oil, exhaust-gas) should be closely monitored, as a NOx system generates a lot of additional heat.

Conclusion

A constant subject of discussion among enthusiasts is the appropriate order of engine mods. It is tempting to do those mods first which give the best performance increase for the money. It is also tempting to do only those mods which provide an immediate power increase, and to leave the "safety mods" for later. A full understanding of detonation in general, and of one's own car in particular, is important to making informed decisions on the appropriate order of engine modifications. At best, unchecked detonation reduces power and efficiency. At worst, it can destroy an engine.